Water-absorbent resin particles are mainly used for disposable diapers, sanitary napkins, incontinence pads, pet sheets, water-retaining materials for soil, water-blocking materials for power cable, dew condensation prevention agents, and the like. Absorbent articles, for example, disposable diapers, are typically formed such that an absorbent material composed of water-absorbent resin particles and hydrophilic fibers is sandwiched between a liquid-permeable sheet to be in contact with the body and a liquid-impermeable sheet provided on the opposite side. The absorbent material is produced, for example, by allowing a mixture including water-absorbent resin particles and crushed hydrophilic fiber to be layered on a metal mesh by means of an air flow, and then, allowing the layered product to be compressed by pressing.
Examples of known water-absorbent resin particles include hydrolysates of starch-acrylonitrile graft copolymers, neutralized products of starch-acrylic acid graft copolymers, saponified products of vinyl acetate-acrylic acid ester copolymers, crosslinked products of partially neutralized polymers of acrylic acid, and partially neutralized polyacrylic acids.
The water-absorbent resin particles used in absorbent materials are required to exhibit not only an excellent water-absorption capacity and water-retention capacity, but also suitable particle size and narrow particle size distribution. When particles with a large particle size are dominantly present, the absorbent material, when compressed, is likely to become hard. When particles with a small particle size are dominantly present, the particles pass through the metallic mesh in the process of producing the absorbent material; thus, such particles are not preferable. Specifically, the water-absorbent resin particles used in absorbent materials are desired to have a median particle size suitable for the design of the intended absorbent materials or absorbent articles, and narrow particle size distribution.
From the standpoint of high performance of the resulting water-absorbent resin particles and the simplicity of the production method, polymerization of a water-soluble ethylenically unsaturated monomer is the mainstream method for producing water-absorbent resin particles. Examples of the polymerization method include an aqueous solution polymerization method comprising polymerizing an aqueous solution of a water-soluble ethylenically unsaturated monomer to obtain a water-containing gel, milling the gel, and drying the gel; and a reversed-phase suspension polymerization method comprising dispersing a water-soluble ethylenically unsaturated monomer in the presence of a dispersion stabilizer in an organic dispersion medium, such as a hydrocarbon dispersion medium, for suspension polymerization to thereby obtain a water containing-gel, dehydrating the gel, and drying the gel.
In the aqueous solution polymerization method, the water-containing gel obtained after polymerization is in the form of viscous block-shaped material, which therefore make the milling step and drying step complicated, increasing the likelihood of the generation of fine particles in the milling step; this lowers the possibility of obtaining water-absorbent resin particles with a suitable particle size and narrow particle size distribution. In the reversed-phase suspension polymerization method, however, it is possible to control the size of the particles by adjusting the size of the droplets of the water-soluble ethylenically unsaturated monomer dispersed in a dispersion medium. Thus, there has been proposed a variety of techniques for controlling the particle size based on the reversed-phase suspension polymerization method.
Examples of proposed techniques for achieving narrow particle size distribution include a polymerization method performed under reduced pressured using a sorbitol fatty acid ester as a dispersion stabilizer (Patent Literature 1), a method using a sorbitan fatty acid ester with an HLB of 8 to 12 as a dispersion stabilizer (Patent Literature 2), and a method using a polyglycerol fatty acid ester as a dispersion stabilizer (Patent Literature 3). However, even these techniques have not been capable to provide water-absorbent resin particles that exhibit satisfactory performance from the standpoint of excellent water-absorption ability, suitable particle size, and narrow particle size distribution.